Re: grand marquis water leak
Wow - my family owned a 1975 Lincoln Continental when I was a kid. Christmas of 1982 was a very green one, and it rained heavily Christmas Eve. I'll never forget it, because my sister stepped into the car and was in almost a foot of water (Lincolns of the 70s had deep foot wells). Like your scenario, the seats weren't wet.
Why am I mentioning this? Well, I know it sounds like spraying the car to try to find the solution sounds like a good one, but you will have a LOT of trouble trying to find the problem. A hose isn't going to emulate mother nature. I say this because my father spent the summer of '83 hosing down that Lincoln (evidentally, the leak was on the passenger side as well) with gallons upon gallons of water. He'd use a sharp stream, and a gentle spray - still no luck. I believe the leak was found by accident one fall morning a year later when there was a similar rain storm. If I recall correctly, rain was getting behind one of the chrome trim pieces, passing through to the inside of the car, and flowing down a pillar into the rear foot well. It wasn't a door or window seal as first suspected.
Unfortunately, none of this is going to help you as there is nothing similar in design to either cars. However, I will offer this advice - pay particular attention to the trim pieces - there's a possibility a similar situation exists where water is getting behind some trim, by-passing window & door seals, and finding it's way into the interior of the car. As for why the car leaks when it rains a particular way, yet is bone dry when you use a hose - only mother nature can answer that. Good luck - hopefully you find the leak...